During the monsoon, or summer thunderstorm season, Arizona experiences more severe weather than many other states. On some occasions, a severe storm may spawn a microburst. More often, high winds, dust and severe downpours resulting in flash floods are common monsoon occurrences.
Prior to 2008, the Phoenix area monsoon was considered to have started when there were three consecutive days when the dew point averaged 55 degrees or higher.
In 2008 the National Weather Service decided to take the guesswork out of monsoon start and end dates. After all, monsoon is a season, and most people should not be concerned with whether or not a particular dust storm was defined as monsoon storm or not.
Beginning in 2008, June 15 was established as the first day of monsoon, and September 30 as the last day. Now we can be more concerned with monsoon safety and less concerned with definitions.
Monsoon Facts for Phoenix
Meteorologists still track and report dew points and study monsoon weather patterns. Here are some technical monsoon facts for the area as reported by the National Weather Service. The facts relate to dew point and the meteorological definition of monsoon, and not the date on the calendar.
- The average starting date of the monsoon in Phoenix is July 7.
- The average ending date of the monsoon is September 13.
- The earliest start date for the monsoon was June 16, 1925.
- The latest start date for the monsoon was July 25, 1987.
- The average date of the first break in the monsoon is August 16.
- The average total number of monsoon days (where a monsoon day is considered one with an average dew point of 55 degrees or higher) is 56.
- The greatest number of monsoon days was 99, recorded in 1984.
- The fewest number of monsoon days was 27, recorded in 1962.
- The greatest number of consecutive monsoon days was 72, from June 25 through September 4, 1984. This was also the greatest number of consecutive days with dew points of 60 degrees or higher.
- In Phoenix, normal rainfall during July, August, and September is 2.65 inches.
- The wettest monsoon occurred in 1984 when we had 9.38 inches of rain.
- The driest monsoon occurred in 1924 with only 0.35 inches.
Find more Phoenix monsoon data at the Arizona State Climate Office.